Study On Emulsification Mechanisms Of Muscle Proteins And Fat During Shearing

Animal fats are an essential raw material in comminuted meat batters and also play important roles in stabilizing meat emulsions, reducing cooking loss and purge loss, improving water holding capacity and providing juiciness and gel hardness. In the past, the research on emulsification properties and mechanisms of muscle proteins was mainly based on meat emulsion model by incorporation of vegetable oils. There are two alternative theories for meat batter stabilization:the oil-in-water emulsion theory, which is based on the formation of an interfacial protein film (IPF) around fat globules, and the physical entrapment theory, which centers on the entrapment of the fat within the protein matrix. However, muscle tissue and fat are always used to be comminuted for meat batters and vegetable oils are scarcely used in meat industry. Until now, the question of emulsification mechanisms of muscle protein and fat when sheared has not yet been resolved. Moreover, liquid and fat losses usually occur together in comminuted meat products. Meat batter is a complex emulsion system and its formation and stabilization are influenced by many factors, such as chopping time, NaCl concentration, fat particle size and distribution, and so on. In order to study the emulsification mechanisms of muscle protein (Longissimus dorsi) and fat (back fat) when directly sheared, fat particle size and distribution, emulsion stability and microstructure of meat emulsions and its emulsion gel were investigated. On this base, the effects of NaCl concentration, pH and fat/muscle protein on the quality property and microstructure of comminuted meat products were also studied. All these results will provide more insight into emulsification mechanisms of muscle protein and quality improvement of comminuted meat products. The detailed content and results are showed as follows.1 Effects of shearing time on particle distribution and emulsification properties of meat batterThe effects of shearing time (1 min,3 min,5 min and 7 min) on particle size and distribution, emulsion stability and gel hardness were investigated at 3000 rpm (round per minute). The results showed that the two emulsion systems of shearing fat for 5 min and 7 min had smaller fat particles, more uniform distribution of fat particles, better emulsion stability and greater emulsion gel strength. But there were no significant difference between these two emulsion systems. It could form a good meat emulsion system by shearing back fat 5min at 3000 rpm in this current study, which basically meets the requirement of the study in the following sections.2、Distribution law of fat particles during the process of shearing meat batterMicrostructures of the raw meat emulsion and its dilution were observed by general microscope, confocal scanning laser microscopy (CSLM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. The results showed that back fat was sheared into solid fat particles and liquid fat droplets of different shapes and sizes, which were surrounded by a layer of protein film on the surfaces. Some fat particles and droplets aggregated and formed larger fat particle mass. Whether individual fat particle (droplet) or fat particle mass was located in protein matrix. They could not deviate from protein matrix and existed alone in the meat emulsion of high viscosity.3 Distribution law of fat particles in meat emulsion gelPhysicochemical and distribution characteristics of fat particles in meat emulsion gel were observed by SEM and TEM, respectively. The results showed that there were fat globules of different sizes and solid fat particles of different shapes in the meat emulsion gel, which were surrounded by a layer of protein film. Some fat globules and particles could crosslink or aggregate into a large fat mass. These fat particles and mass were distributed in the protein gel matrix. Moreover, some fat globules, particles and mass were partly/wholly embedded in protein gel matrix. The above-mentioned interactions kept the fat globules/particles in the meat emulsion gel stabilized, which has realized the emulsification of muscle protein and fat in comminuted meat products.4 Effects of NaCl concentration and pH on amount of protein adsorbed by fat particles and emulsion gel propertyThe effects of NaCl concentration (0.2 mol/L and 0.6 mol/L) and pH (6.0 and 6.5) on amount of protein adsorbed by fat particles, amount of protein per unit surface, emulsion stability, gel hardness and microstructure of meat emulsion were investigated. The results showed that higher concentration (0.6 mol/L) NaCl solution cloud significantly increase the total amount of protein adsorbed, amount of protein per unit surface, emulsion stability, emulsion gel hardness and microstructure of meat emulsion (P<0.05). At the same concentration (0.2 mol/L or 0.6 mol/L) NaCl solution, pH value (pH6.5) could enhance the above-mentioned indexes s effectively (P>0.05). Moreover, separation and extraction of the proteins adsorbed by fat particles were performed by the methods of centrifugation and freezing and thawing. The results of sodium dodecylsulfate polyacrylamide gel electrophoresis (SDS-PAGE) showed that protein film around fat particles were made up of myosin, actin, and propomyosin, troponin, C-protein, and so on.5 Effects of fat/protein ration on quality and rheological property of meat battersThe effects of fat/protein (FP) on emulsion stability, color, pH, texture property, rheological property and microstructure of meat emulsion and its emulsion gel were investigated. The results showed that percentages of the total expressible fluid and fat loss in higher FP (29:13) treatment were significantly more than those of middle FP (18:12) and lower FP (9:12) treatments (P<0.05), but there was no significant difference between the middle and lower FP treatments (P>0.05). With increasing the FP ratio, the L*-value of the uncooked meat emulsion and its gel significantly increased (P<0.05), while a* and b* values significantly decreased (P<0.05) and pH value also decreased (P>0.05). The pH and a* values of the gels were larger than those of the uncooked meat emulsions, while the results of the L*-and b*-values were opposite. The gel hardness of the higher FP treatment was significantly higher those of the middle and lower treatments (P<0.05), while its springiness was significantly lower than those of the middle and lower FP treatments (P<0.05). As for chewiness and gumminess of gels, the middle FP treatment was the highest level and the lower FP treatment was in the second place, and the higher FP treatment was the lowest level. The storage modulus (G’) of meat emulsion was related to the FP values. The lower the FP value, the less the storage modulus. The higher the FP value, the more the storage modulus. Moreover, the middle and the gels of lower FP treatment had more compact, smooth and uniform protein matrix, while the gels of higher FP treatment had coarse protein matrix and also some residual protein films.To sum up, animal fat could be sheared into liquid fat globules of different sizes and solid fat particles of different shapes when meat batters were chopped sufficiently in high concentration NaCl and pH (e.g.0.6 mol/L NaCl-pH 6.5) solution. These fat globules and solid fat particles were surrounded by a layer of protein film. During heat-induced process, protein films could denature. Most fat globules and particles of proper sizes were still surrounded by protein film. Some fat globules and particles could crosslink or aggregate into a large fat mass. These fat globules, particles and mass were distributed in the protein gel matrix. And some fat globules, particles or mass could be physically embedded in protein gel matrix. These interactions have realized the emulsification of muscle protein and fat in comminuted meat products. Moreover, NaCl concentration, pH and fat/protein could influence emulsion stability and quality property of emulsion gel.